Background

Pete Scandlon of Pete's Pet Shop is interested in using Ethereum as an efficient way to handle their pet adoptions. The store has space for 16 pets at a given time, and they already have a database of pets. As an initial proof of concept, Pete wants to see a dapp which associates an Ethereum address with a pet to be adopted.

The website structure and styling will be supplied. Our job is to write the smart contract and front-end logic for its usage.

Setting up the development environment

There are a few technical requirements before we start. Please install the following:

Once we have those installed, we only need one command to install Truffle:

npm install -g truffle

To verify that Truffle is installed properly, type truffle version on a terminal. If you see an error, make sure that your npm modules are added to your path.

We also will be using Ganache, a personal blockchain for Ethereum development you can use to deploy contracts, develop applications, and run tests. You can download Ganache by navigating to http://truffleframework.com/ganache and clicking the "Download" button.

Note: If you are developing in an environment without a graphical interface, you can also use Truffle Develop, Truffle's built-in personal blockchain, instead of Ganache. You will need to change some settings---such as the port the blockchain runs on---to adapt the tutorial for Truffle Develop.

Creating a Truffle project using a Truffle Box

Truffle initializes in the current directory, so first create a directory in your development folder of choice and then moving inside it.

mkdir pet-shop-tutorial
cd pet-shop-tutorial

We've created a special Truffle Box just for this tutorial called pet-shop, which includes the basic project structure as well as code for the user interface. Use the truffle unbox command to unpack this Truffle Box.

truffle unbox pet-shop

Note: Truffle can be initialized a few different ways. Another useful initialization command is truffle init, which creates an empty Truffle project with no example contracts included. For more information, please see the documentation on Creating a project.

Directory structure

The default Truffle directory structure contains the following:

contracts/: Contains the Solidity source files for our smart contracts. There is an important contract in here called Migrations.sol, which we'll talk about later.

migrations/: Truffle uses a migration system to handle smart contract deployments. A migration is an additional special smart contract that keeps track of changes.

The pet-shop Truffle Box has extra files and folders in it, but we won't worry about those just yet.

Writing the smart contract

We'll start our dapp by writing the smart contract that acts as the back-end logic and storage.

Create a new file named Adoption.sol in the contracts/ directory.

Add the following content to the file:

pragma solidity ^0.5.0;
contract Adoption {
}

Things to notice:

The minimum version of Solidity required is noted at the top of the contract: pragma solidity ^0.5.0;. The pragma command means "additional information that only the compiler cares about", while the caret symbol (^) means "the version indicated or higher".

Like JavaScript or PHP, statements are terminated with semicolons.

Variable setup

Solidity is a statically-typed language, meaning data types like strings, integers, and arrays must be defined. Solidity has a unique type called an address. Addresses are Ethereum addresses, stored as 20 byte values. Every account and smart contract on the Ethereum blockchain has an address and can send and receive Ether to and from this address.

Add the following variable on the next line after contract Adoption {.

address[16] public adopters;

Things to notice:

We've defined a single variable: adopters. This is an array of Ethereum addresses. Arrays contain one type and can have a fixed or variable length. In this case the type is address and the length is 16.

You'll also notice adopters is public. Public variables have automatic getter methods, but in the case of arrays a key is required and will only return a single value. Later, we'll write a function to return the whole array for use in our UI.

Your first function: Adopting a pet

Let's allow users to make adoption requests.

Add the following function to the smart contract after the variable declaration we set up above.

In Solidity the types of both the function parameters and output must be specified. In this case we'll be taking in a petId (integer) and returning an integer.

We are checking to make sure petId is in range of our adopters array. Arrays in Solidity are indexed from 0, so the ID value will need to be between 0 and 15. We use the require() statement to ensure the ID is within range.

If the ID is in range, we then add the address that made the call to our adopters array. The address of the person or smart contract who called this function is denoted by msg.sender.

Finally, we return the petId provided as a confirmation.

Your second function: Retrieving the adopters

As mentioned above, array getters return only a single value from a given key. Our UI needs to update all pet adoption statuses, but making 16 API calls is not ideal. So our next step is to write a function to return the entire array.

Add the following getAdopters() function to the smart contract, after the adopt() function we added above:

Since adopters is already declared, we can simply return it. Be sure to specify the return type (in this case, the type for adopters) as address[16] memory. memory gives the data location for the variable.

The view keyword in the function declaration means that the function will not modify the state of the contract. Further information about the exact limits imposed by view is available here.

Compiling and migrating the smart contract

Now that we have written our smart contract, the next steps are to compile and migrate it.

Truffle has a built-in developer console, which we call Truffle Develop, which generates a development blockchain that we can use to test deploy contracts. It also has the ability to run Truffle commands directly from the console. We will use Truffle Develop to perform most of the actions on our contract in this tutorial.

Compilation

Solidity is a compiled language, meaning we need to compile our Solidity to bytecode for the Ethereum Virtual Machine (EVM) to execute. Think of it as translating our human-readable Solidity into something the EVM understands.

In a terminal, make sure you are in the root of the directory that contains the dapp and type:

Migration

Now that we've successfully compiled our contracts, it's time to migrate them to the blockchain!

A migration is a deployment script meant to alter the state of your application's contracts, moving it from one state to the next. For the first migration, you might just be deploying new code, but over time, other migrations might move data around or replace a contract with a new one.

You'll see one JavaScript file already in the migrations/ directory: 1_initial_migration.js. This handles deploying the Migrations.sol contract to observe subsequent smart contract migrations, and ensures we don't double-migrate unchanged contracts in the future.

Now we are ready to create our own migration script.

Create a new file named 2_deploy_contracts.js in the migrations/ directory.

Before we can migrate our contract to the blockchain, we need to have a blockchain running. For this tutorial, we're going to use Ganache, a personal blockchain for Ethereum development you can use to deploy contracts, develop applications, and run tests. If you haven't already, download Ganache and double click the icon to launch the application. This will generate a blockchain running locally on port 7545.

You can see the migrations being executed in order, followed by some information related to each migration. (Your information will differ.)

In Ganache, note that the state of the blockchain has changed. The blockchain now shows that the current block, previously 0, is now 4. In addition, while the first account originally had 100 ether, it is now lower, due to the transaction costs of migration. We'll talk more about transaction costs later.

Ganache after migration

You've now written your first smart contract and deployed it to a locally running blockchain. It's time to interact with our smart contract now to make sure it does what we want.

Testing the smart contract

Truffle is very flexible when it comes to smart contract testing, in that tests can be written either in JavaScript or Solidity. In this tutorial, we'll be writing our tests in Solidity.

DeployedAddresses.sol: When running tests, Truffle will deploy a fresh instance of the contract being tested to the blockchain. This smart contract gets the address of the deployed contract.

Adoption.sol: The smart contract we want to test.

Note: The first two imports are referring to global Truffle files, not a truffle directory. You should not see a truffle directory inside your test/ directory.

Then we define three contract-wide variables:

First, one containing the smart contract to be tested, calling the DeployedAddresses smart contract to get its address.

Second, the id of the pet that will be used to test the adoption functions.

Third, since the TestAdoption contract will be sending the transaction, we set the expected adopter address to this, a contract-wide variable that gets the current contract's address.

Testing the adopt() function

To test the adopt() function, recall that upon success it returns the given petId. We can ensure an ID was returned and that it's correct by comparing the return value of adopt() to the ID we passed in.

Add the following function within the TestAdoption.sol smart contract, after the declaration of Adoption:

We call the smart contract we declared earlier with the ID of expectedPetId.

Finally, we pass the actual value, the expected value and a failure message (which gets printed to the console if the test does not pass) to Assert.equal().

Testing retrieval of a single pet's owner

Remembering from above that public variables have automatic getter methods, we can retrieve the address stored by our adoption test above. Stored data will persist for the duration of our tests, so our adoption of pet expectedPetId above can be retrieved by other tests.

Add this function below the previously added function in TestAdoption.sol.

// Testing retrieval of a single pet's owner
function testGetAdopterAddressByPetId() public {
address adopter = adoption.adopters(expectedPetId);
Assert.equal(adopter, expectedAdopter, "Owner of the expected pet should be this contract");
}

After getting the adopter address stored by the adoption contract, we assert equality as we did above.

Testing retrieval of all pet owners

Since arrays can only return a single value given a single key, we create our own getter for the entire array.

Add this function below the previously added function in TestAdoption.sol.

Note the memory attribute on adopters. The memory attribute tells Solidity to temporarily store the value in memory, rather than saving it to the contract's storage. Since adopters is an array, and we know from the first adoption test that we adopted pet expectedPetId, we compare the testing contracts address with location expectedPetId in the array.

Creating a user interface to interact with the smart contract

Now that we've created the smart contract, deployed it to our local test blockchain and confirmed we can interact with it via the console, it's time to create a UI so that Pete has something to use for his pet shop!

Included with the pet-shop Truffle Box was code for the app's front-end. That code exists within the src/ directory.

The front-end doesn't use a build system (webpack, grunt, etc.) to be as easy as possible to get started. The structure of the app is already there; we'll be filling in the functions which are unique to Ethereum. This way, you can take this knowledge and apply it to your own front-end development.

Instantiating web3

Open /src/js/app.js in a text editor.

Examine the file. Note that there is a global App object to manage our application, load in the pet data in init() and then call the function initWeb3(). The web3 JavaScript library interacts with the Ethereum blockchain. It can retrieve user accounts, send transactions, interact with smart contracts, and more.

Remove the multi-line comment from within initWeb3 and replace it with the following:

First, we check if we are using modern dapp browsers or the more recent versions of MetaMask where an ethereum provider is injected into the window object. If so, we use it to create our web3 object, but we also need to explicitly request access to the accounts with ethereum.enable().

If the ethereum object does not exist, we then check for an injected web3 instance. If it exists, this indicates that we are using an older dapp browser (like Mist or an older version of MetaMask). If so, we get its provider and use it to create our web3 object.

If no injected web3 instance is present, we create our web3 object based on our local provider. (This fallback is fine for development environments, but insecure and not suitable for production.)

Instantiating the contract

Now that we can interact with Ethereum via web3, we need to instantiate our smart contract so web3 knows where to find it and how it works. Truffle has a library to help with this called truffle-contract. It keeps information about the contract in sync with migrations, so you don't need to change the contract's deployed address manually.

Still in /src/js/app.js, remove the multi-line comment from within initContract and replace it with the following:

We first retrieve the artifact file for our smart contract. Artifacts are information about our contract such as its deployed address and Application Binary Interface (ABI). The ABI is a JavaScript object defining how to interact with the contract including its variables, functions and their parameters.

Once we have the artifacts in our callback, we pass them to TruffleContract(). This creates an instance of the contract we can interact with.

With our contract instantiated, we set its web3 provider using the App.web3Provider value we stored earlier when setting up web3.

We then call the app's markAdopted() function in case any pets are already adopted from a previous visit. We've encapsulated this in a separate function since we'll need to update the UI any time we make a change to the smart contract's data.

Getting The Adopted Pets and Updating The UI

Still in /src/js/app.js, remove the multi-line comment from markAdopted and replace it with the following:

We access the deployed Adoption contract, then call getAdopters() on that instance.

We first declare the variable adoptionInstance outside of the smart contract calls so we can access the instance after initially retrieving it.

Using call() allows us to read data from the blockchain without having to send a full transaction, meaning we won't have to spend any ether.

After calling getAdopters(), we then loop through all of them, checking to see if an address is stored for each pet. Since the array contains address types, Ethereum initializes the array with 16 empty addresses. This is why we check for an empty address string rather than null or other falsey value.

Once a petId with a corresponding address is found, we disable its adopt button and change the button text to "Success", so the user gets some feedback.

Any errors are logged to the console.

Handling the adopt() Function

Still in /src/js/app.js, remove the multi-line comment from handleAdopt and replace it with the following:

We use web3 to get the user's accounts. In the callback after an error check, we then select the first account.

From there, we get the deployed contract as we did above and store the instance in adoptionInstance. This time though, we're going to send a transaction instead of a call. Transactions require a "from" address and have an associated cost. This cost, paid in ether, is called gas. The gas cost is the fee for performing computation and/or storing data in a smart contract. We send the transaction by executing the adopt() function with both the pet's ID and an object containing the account address, which we stored earlier in account.

The result of sending a transaction is the transaction object. If there are no errors, we proceed to call our markAdopted() function to sync the UI with our newly stored data.

Interacting with the dapp in a browser

Now we're ready to use our dapp!

Installing and configuring MetaMask

The easiest way to interact with our dapp in a browser is through MetaMask, a browser extension for both Chrome and Firefox.

Install MetaMask in your browser.

Once installed, you'll see the MetaMask fox icon next to your address bar. Click the icon and you'll see this screen appear:

Privacy Notice

Click Accept to accept the Privacy Notice.

Then you'll see the Terms of Use. Read them, scrolling to the bottom, and then click Accept there too.

Click the left-pointing arrow next to "Settings" to close out of the page and return to the Accounts page.

Each account created by Ganache is given 100 ether. You'll notice it's slightly less on the first account because some gas was used when the contract itself was deployed and when the tests were run.

MetaMask account configured

Configuration is now complete.

Installing and configuring lite-server

We can now start a local web server and use the dapp. We're using the lite-server library to serve our static files. This shipped with the pet-shop Truffle Box, but let's take a look at how it works.

Open bs-config.json in a text editor (in the project's root directory) and examine the contents:

{
"server": {
"baseDir": ["./src", "./build/contracts"]
}
}

This tells lite-server which files to include in our base directory. We add the ./src directory for our website files and ./build/contracts directory for the contract artifacts.

We've also added a dev command to the scripts object in the package.json file in the project's root directory. The scripts object allows us to alias console commands to a single npm command. In this case we're just doing a single command, but it's possible to have more complex configurations. Here's what yours should look like:

This tells npm to run our local install of lite-server when we execute npm run dev from the console.

Using the dapp

Start the local web server:

npm run dev

The dev server will launch and automatically open a new browser tab containing your dapp.

Pete's Pet Shop

To use the dapp, click the Adopt button on the pet of your choice.

You'll be automatically prompted to approve the transaction by MetaMask. Click Submit to approve the transaction.

Adoption transaction review

You'll see the button next to the adopted pet change to say "Success" and become disabled, just as we specified, because the pet has now been adopted.

Adoption success

Note: If the button doesn't automatically change to say "Success", refreshing the app in the browser should trigger it.

And in MetaMask, you'll see the transaction listed:

MetaMask transaction

You'll also see the same transaction listed in Ganache under the "Transactions" section.

Congratulations! You have taken a huge step to becoming a full-fledged dapp developer. For developing locally, you have all the tools you need to start making more advanced dapps. If you'd like to make your dapp live for others to use, stay tuned for our future tutorial on deploying to the Ropsten testnet.